1. Field of the Invention
The present invention relates to a microelectronic device for emitting electrons by using a vacuum microelectronic technique, and a method of manufacturing the same.
2. Description of the Related Art
With recent advancement in semiconductor micropatterning techniques, micron-order vacuum tubes have been developed. The purpose of this development is to reconsider a vacuum as an electron transportation medium, thereby developing an ultra-high-speed, environment-resistant, electron emitting device which overcomes the drawbacks of vacuum tubes replaced by solid-state devices.
Typical electron emitting devices now being developed are of a Spindt type (Gray type), a plane type, and an MIM (Metal-Insulator-Metal).
In a Spindt type electron emitting device, as shown in FIG. 45, an emitter electrode 1 extends substantially vertically from a substrate 2 in the form of a quadrangular prism or a cone. In a plane type electron emitting device, as shown in FIGS. 46A and 46B, an emitter electrode 3 extends in a direction parallel to a substrate 2 in the form of a triangular diving platform, i.e. a wedge. In FIGS. 45, 46A and 46B, reference numerals 4 and 5 denote gate electrodes for extracting electrons from the emitter electrodes 1 and 3.
Specifically, the Spindt type and plane type electron emitting devices have emitter electrodes 1 and 3 with sharpened tip portions. An electric field is applied to the emitter electrode 1, 3 from the adjacent gate electrode 4, 5, thereby extracting (discharging) electrons from the emitter electrode 1, 3.
As disclosed in, for example, J. EE Japan, Vol. 112, No. 4 (1992), pp. 257-262 by Kuniyoshi Yokoh in the Electrical Communication Laboratory of Tohoku University, a Spindt type electron emitting device may be manufactured by a technique of obliquely depositing a cathode chip while rotating a substrate, which technique was developed by C. A. Spindt et al. in Stanford Laboratory, or by a technique of performing selective anisotropic etching of an Si single crystal, which technique was developed by H. P. Gray et al. in the U.S. Navy Laboratory.
Methods of manufacturing other types of emitter electrodes of the plane type device, etc. are explained, for example, in "Application of Small Cold Cathode--Vacuum Microelectronic Device--" (OPTRONICS, No. 109 (1991), pp. 193-198) and "Experimental Manufacture and Application of Small-Sized Triode Vacuum Tube" (the 11th Laboratory Reference for 132nd Comittee of Japan Society for the Promotion of Science (1990), pp. 7-13 for "Industrial Application of Charged") by Junji Itoh and Seigo Kanemaru of Kogyo Gijutsuin Denshigijutsu Sogo Kenkyujo (the Electronics Research Center of the Agency of Industrial Science and Technology).
On the other hand, in an MIM type electron emitting device, although not shown, a thin insulating film and a thin conductor film are laminated on a surface of a conductor which will become an emitter electrode. An intense electric field is applied to the surface of the emitter electrode from the conductor film, thereby extracting electrons with use of quantum-mechanical tunneling phenomenon.
Whether the development of such a device is significant depends on how much the operating voltage of the device can be decreased. In order to decrease the operating voltage, it is necessary to enhance the electron emission efficiency (emission current density) of the emitter electrode of the electron emitting device.
There is an idea that the electron emitting device is applicable to an electron emission source of an electron beam plotter or a planar display. For this purpose, it is desirable to emit electrons at high density in a planar manner.
In the Spindt type and plane type devices, the emitter electrode is formed in a pyramidal or conical shape (Spindt type) or in a wedge shape (plane type). Thereby, a tip portion of the emitter electrode is sharpened and the electron emission efficiency is enhanced.
However, since the emitter electrode 1 and 3 of the conventional electron emitting device has a pyramidal, conical or wedge-like shape, as mentioned above, the interval of field electron emission devices is limited by the size of the bottom surface of the emitter electrode 1 and 3. Thus, it is difficult to increase the density of electron emitting devices. Since the density of electrons emitted from the electron emitting device (i.e. magnitude of emission current) is influenced by the number of emitter electrodes 1 and 3, it is also difficult to increase the emission current per unit area.
In order to obtain a higher emission current with a lower drive voltage in the electron emitting device, it is necessary to sharpen the tip portion of the emitter electrode as much as possible, thereby increasing the degree of concentration of electric field.
In the case of conventional electron emitting devices, however, the emitter electrode is sharpened by etching or superposition exposure. Thus, a complex process is needed to sharpen the emitter electrode, and it is difficult to sharpen the emitter electrode. Furthermore, since the process for manufacturing the emitter electrode is complex, the reproducibility is low and it is difficult to uniformly produce a great number of emitter electrodes.
Besides, the degree of sharpness of the emitter electrode depends on the resolution of an exposure apparatus to be employed.
Specifically, the precision of the shape of the tip of the emitter electrode depends on, for example, the resolution of a stepper for performing mask patterning. Since the resolution of the apparatus is limited, the attainable degree of sharpness of the emitter electrode and the degree of density of electrodes are limited to a certain level.
On the other hand, with respect to the MIM type electron discharge device, there is no need to sharpen the electrode. In the above-described Spindt type device, electrons cannot be emitted in a planar fashion unless the emitter electrodes are formed at high density. In the MIM type device, however, electrons (or an electron beam) can be emitted in a planar fashion, irrespective of the density of the formation of electrodes. Since there is no need to sharpen the emitter electrode, the production of the emitter electrode is very easy and the yield of electrodes is high.
However, in order to enhance the electron emission efficiency of MIM type electron emitting devices, it is necessary to decrease the thickness of the insulating film as much as possible and to decrease the distance (gap) between the surface of the emitter electrode and the conductor film.
When the thickness of the insulating film cannot be thinned, the electron emission efficiency deteriorates and it is necessary to produce a high potential difference between the conductor film and the emitter electrode. As a result, the operating voltage increases.
In order to avoid such inconvenience, the thickness of the insulating film must be decreased to, e.g. 100 .ANG.. However, it is very difficult to form the insulating film with such a thickness, since no lattice defect is permitted to be present in the insulating film.